OTDRs are used in the field of fiber optic communication to gain insight into the main optical parameters of a fiber optic transmission system, like information regarding fiber attenuation, and to detect impairments or irregularities, like deteriorated connectors or fiber breakages.
In fiber optic communications, an electromagnetic wave—also called lightwave—propagates through an optical fiber and thus transmits information. For this purpose, the lightwave is modulated, which means that a physical parameter of the lightwave is varied depending on an information carrying signal. In a stricter sense, the term “light” denotes electromagnetic radiation within a wavelength range with high sensitivity of the human eye. Typically, the human eye is sensitive to radiation from 400 nm to 780 nm. In physics, the term “light” is sometimes used in a broader sense and might refer to any kind of electromagnetic radiation, whether visible or not. Fiber optic communication usually makes use of electromagnetic radiation in the infrared range that is not visible to the human eye. Commonly, electromagnetic waves propagating along the fiber axis are denoted as lightwaves. In the following, the terms “light” and “lightwave” will refer to any kind of electromagnetic wave that can be guided within an optical fiber irrespective of its wavelength.
An OTDR typically comprises a light source and a detector. The light source sends optical pulses into the fiber optic transmission system, which in the course of propagation in the fiber experience attenuation and are continuously reflected back towards the light source. This is due to irregularities and impurities inside the fiber that cause the light to be redirected in different directions creating both signal attenuation and backscattering, known as Rayleigh backscattering. Rayleigh backscattering can be used to calculate the level of attenuation in the fiber as a function of fiber distance.
Reflected optical pulses are received by the detector, which records the part of the optical power reflected back together with the corresponding delay in the form of a so-called OTDR trace. An OTDR trace typically characterizes the absorption properties of a fiber optic transmission system by registering the power of the reflected light as a function of distance along the fiber optic transmission system upon assuming constant propagation velocity of the optical signals. Whenever the amount of back reflected light changes abruptly at a given location of a fiber optic transmission system, this change can be noticed in the corresponding OTDR trace, generally in the form of a sudden increase or a sudden drop of back reflected power.
The analysis of OTDR traces may hence help to detect deficiencies in a fiber optic transmission system such as splices or connectors providing increased loss, reflection points, or other kind of irregularity. While OTDRs work acceptably well in fiber optic transmission systems with uniform fiber characteristics, a non-negligible risk of wrong results exists in the case of fiber optic transmission systems composed of different fiber types. A change in fiber type is typically associated with a change in the so-called mode field diameter of the fiber.
The mode field diameter or mode field area is a measure of the radial extent of the optical intensity (i.e. the optical power per unit area) distribution of a mode across a single-mode fiber. For example in the case of a Gaussian intensity distribution, the electric and magnetic field strengths are reduced to 1/e of the maximum values at locations in a plane perpendicular to the fiber axes for which the distance to the intersection of the center of the fiber core with this plane corresponds to half of the mode field diameter, i.e. to the mode field radius. In other words, the power density at the mode field diameter is reduced to 1/e2 of the maximum power density.
The amount of power continuously reflected back in a fiber optic transmission system due to Rayleigh backscattering depends on several physical parameters such as the effective refractive index and the scattering coefficient, but also shows a strong dependence on the mode field diameter. In particular, the backscattering factor scales inversely with the square of the mode field diameter, i.e. inversely with the mode field area. In practical OTDR measurement terms, backscattering characteristics are mostly influenced by the refractive index profile and geometrical properties of the fiber. Since different fiber types usually have different mode field diameters, a change in fiber type along the optical path of an optical signal being transmitted in a fiber optic transmission system results in a change in the amount of backscattered light that is primarily due to the corresponding change in mode field diameter. In particular, a connection between different fiber types may result in a decrease in the mode field diameter, and hence in an increase of the amount of light backscattered. This situation is often referred to as a “gainer” in the art and is a clear indication of such a connection between different fiber types, for a splice between identical fibers never results in such an increase.
However, depending on the concrete change in a mode field diameter, a connection between different fiber types may also result in an increase in the mode field diameter, and hence in a decrease in the amount of light backscattered, and in particular in a decrease which is bigger than the decrease that would normally be caused by actual attenuation at that connection or splice. Such cases are commonly referred to as “exaggerated loss” and make a correct determination of the fiber attenuation difficult. This is due to the fact that in many cases a significant if not predominant part of detected change in the amount of backscattered light is actually due to the change in the mode field diameter and not to events causing actual attenuation, which may play a minor role. Therefore, the analysis of the corresponding OTDR trace may lead to inaccurate results and/or to a wrong interpretation, such as an over-estimation of the true increase in absorption losses. Consequently, a connection between different fiber types may erroneously be reported as a bad quality connection or even as an irregularity in the fiber optic transmission system, while the true cause of the apparent increase of loss is actually an increase of mode field diameter of the fibers involved.
In order to avoid such wrong diagnostics in the analysis of OTDR traces, averaging based on bidirectional OTDR traces taken from the two end points of a fiber optic transmission system is recommended. However, such solutions are only applicable to short optical links as long as access to both ends of the link is possible and there is a sufficient overlap of the respective measurement ranges. However, a typical single-span submarine fiber optic transmission system comprises submarine cables carrying several optical fibers and extends over e.g. 300 km or more. This makes measuring the same part of a fiber optic transmission system from both sides impossible. Further, many such systems comprise remote optical pumped amplifiers (ROPAs), and parts of the system may be difficult to access. Bidirectional OTDR measurements are not possible in fiber optic transmission systems comprising ROPAs even if the ROPAs do not include an isolator. This is due to the fact that unpumped erbium-doped fiber (EDF) coils introduce significant losses in the wavelength range of the transmitted signals.
In addition, terrestrial fibers are typically connected to submarine fibers at landing stations so that direct access to the submarine fibers is very often not possible. For example, in regions close to landing stations, an optical cable is typically buried in the ground in order to avoid the high risk of fiber breakage to occur in shallow water. In addition, the type of fiber used for terrestrial connections is quite often different from that of fibers used for submarine connections. However, characterizing those parts of a fiber optic transmission system close to the landing stations is of major importance. This increases the need of accurate and reliable interpretation of unidirectional OTDR measurements, which is currently not possible in most long-haul fiber optic transmission systems.
Live video analysis of fiber alignment during splicing has been suggested as a possible alternative solution. However, this procedure is not only costly and technically involved but also not applicable to already installed fiber optic transmission systems.
In view of the above, there is room for technical improvements in the determination and analysis of events causing actual attenuation like splice losses and changes in absorption in unidirectional OTDR measurements.